Fix a bug with app_voicemail when trying to use app_directory to leave messages

to another user (options 3, 5, 2).

If the context/extension didn't exist in the dialplan (and why should it have to?),
it would fail, saying that it's an "invalid extension".

(issue BE-71)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@40426 65c4cc65-6c06-0410-ace0-fbb531ad65f3